Various types of vacuum drive rollers are known in the art, the function of such rollers being to prevent slippage between a web and the roller, thus controlling the linear speed of the web.
U.S. Pat. No. 3,630,424, issued Dec. 28, 1971 to John A. Rau discloses a hollow drum connected to a vacuum line and having a pattern of holes formed in the peripheral surface thereof.
U.S. Pat. No. 4,029,249, issued June 14, 1977 to Peter Nagel et al discloses a hollow roller having a surface containing intersecting grooves with holes formed in the roller wall at the intersecting points of the grooves.
The above prior rollers have holes which extend through the roller wall and which are distributed over the entire peripheral surface of the roller. In operation, the web being transported by the roller overlies only a limited number of holes. Since a majority of the holes are open to the atmosphere, an exceptionally high vacuum must be maintained within the roller in order to effectively draw the web against the roller surface. This results not only in a loss of energy but, also, limits the total tension which can be applied to the web in order to prevent possible collapse of the roller. In other conventional rollers a deckling arrangement is used to selectively evacuate only a desired surface area of the roller. Inherent in such arrangements is a certain amount of leakage and, also, there is a limit to the vacuum which can be applied to prevent possible collapse of the roller. In any case, the holes in the prior rollers eventually become clogged by dirt and other foreign matter, requiring periodic removal of the roller for cleaning, with a consequent loss of machine operating time.
A vacuum roller made in accordance with this invention overcomes the shortcomings of prior such devices. It includes a plurality of air permeable members constructed and arranged so that the volume to be evacuated, per unit surface area of the roller, is minimal. Also, the area to be evacuated is immediately adjacent to the web. Thus, a higher vacuum can be utilized to apply a higher tension to the web without fear of the roller collapsing. Also, the roller is so constructed and arranged that the air permeable members are alternately connected to a vacuum line to draw the web against the roller surface, and to an air pressure line to blow out foreign matter from such members.